The present invention relates to a power distribution control apparatus for a four-wheel drive vehicle.
Systems adopted in typical four-wheel drive vehicles include a part-time four-wheel drive system, in which four-wheel drive (4WD) and two-wheel drive (2WD) are switched, and a full-time four-wheel drive system, in which all four wheels are constantly driven. Generally, the part-time four-wheel drive system allows a driver to manually switch the driving mode to the four-wheel drive to directly couple the front and rear wheels. The full-time four-wheel drive system has a center differential between the front and rear wheels, which permits the front and rear wheels to travel at different rates to achieve permanent four-wheel drive.
In addition to the part-time and full-time four-wheel drive systems, a stand-by four-wheel drive system is known. The stand-by four-wheel drive system shifts the driving mode between the four-wheel drive and the two-wheel drive on demand. The stand-by four-wheel drive system includes main drive wheels, which are directly coupled with an internal combustion engine, and sub drive wheels, which are coupled with the internal combustion engine with a coupling. Distribution of power to the sub drive wheels is optimized by changing the engaging force of the coupling in accordance with the road condition and the driving condition.
A power distribution control apparatus of a stand-by four-wheel drive vehicle compares the opening degree of a throttle located in an internal combustion engine, the differential rotation speed between the front and rear wheels (difference between the wheel speed of the front and rear wheels), and variation in the wheel speed of four-wheels (variation of the wheel speed per unit of time) with determination threshold values. If each of the opening degree of the throttle, the differential rotation speed, and the variation of the wheel speed is less than or equal to the corresponding determination threshold value, the power distribution control apparatus determines that the vehicle is in a steady driving state. If it is determined that the vehicle is in the steady driving state, the power distribution control apparatus controls a coupling to reduce distribution of the power to the sub drive wheels (front or rear wheels).
As a result, when it is determined that the vehicle is in the steady driving state, power need not be applied to the sub drive wheels. When power is not applied to the sub drive wheels, transmission loss at, for example, the coupling and the differential gear of the sub-drive wheels is eliminated, which reduces the fuel consumption.
However, in the conventional stand-by four-wheel drive vehicle, the power distribution control apparatus determines that the vehicle is in the steady driving state when the throttle opening is less than or equal to a predetermined determination threshold value regardless of the vehicle speed.
Therefore, if the determination threshold value of the throttle opening is set corresponding to a middle speed range (for example, 40 km/h to 70 km/h) that require high response for stable steering, the steady driving state at a high speed range (for example, 100 km/h) is not easily determined. That is, in this case, the throttle opening in the high speed range is greater than that in the middle speed range. Thus, although the vehicle is in the steady driving state in the high speed range, the throttle opening is greater than or equal to the determination threshold value, which is set corresponding to the middle speed range. Therefore, it is determined that the vehicle is not in the steady driving state.
If a certain determination threshold value of the throttle opening degree is determined regardless of the vehicle speed, it is determined that the vehicle is in the steady driving state when the vehicle is driving at high speed on a flat road. However, in this case, when the vehicle is in the steady driving state on a gentle slope, it is not determined that the vehicle is in the steady driving state, and the determination of the steady driving state is cancelled.
Therefore, in this case, the vehicle is driven by four-wheel drive, which deteriorates fuel consumption.
Accordingly, it is an objective of the present invention to provide a power distribution control apparatus for a four-wheel drive vehicle that correctly determines whether the vehicle is in a steady driving state.
To achieve the above objective, the present invention provides a power distribution control apparatus for a four-wheel drive vehicle. The vehicle has front wheels and rear wheels driven by an engine and a coupling for changing the distribution ratio of torque to the front wheels and the rear wheels. The distribution ratio of torque to the front wheels and the rear wheels is determined by the engaging force of the coupling. The apparatus includes vehicle speed detecting means, a throttle opening degree sensor, and a controller. The vehicle speed detecting means detects the speed of the vehicle. The throttle opening degree sensor detects the throttle opening degree, which is the opening degree of a throttle valve located in the engine. The controller controls the coupling. The controller sets at least one determination threshold value for the throttle opening degree in accordance with the detected vehicle speed, determines the driving state of the vehicle by comparing the detected throttle opening degree with the set determination threshold value, and controls the engaging force of the coupling in accordance with the determined driving state.
The present invention also provides a power distribution controlling method for a four-wheel drive vehicle. The vehicle has front wheels and rear wheels driven by an engine and a coupling for changing the distribution ratio of torque to the front wheels and the rear wheels. The distribution ratio of torque to the front wheels and the rear wheels is determined by the engaging force of the coupling. The method includes: detecting the speed of the vehicle; detecting the throttle opening degree, which is the opening degree of a throttle valve located in the engine; setting at least one determination threshold value for the throttle opening degree in accordance with the detected vehicle speed; determining the driving state of the vehicle by comparing the detected throttle opening degree with the set determination threshold value; and controlling the engaging force of the coupling in accordance with the determined driving state.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.